Resonant converter circuit and resonant converter circuit control method

ABSTRACT

A resonant converter circuit comprises a multi-level inverter circuit placed before a resonant unit, and the multi-level inverter circuit can reduce a voltage to be input to the resonant unit. The reduced input voltage of the resonant unit results in a drop in an output voltage of the entire resonant converter circuit. In this process, the final output voltage is adjusted by adjusting the input voltage of the resonant unit, with no need to substantially adjust a switching frequency of the resonant converter circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CN2018/122306, filed on Dec. 20, 2018, which claims priority toChinese Patent Application No. 201711407832.5, filed on Dec. 22, 2017.The disclosures of the aforementioned applications are herebyincorporated by reference in their entireties.

TECHNICAL FIELD

The present application relates to the field of switch-mode power supplytechnologies, and in particular, to a resonant converter circuit and aresonant converter circuit control method.

BACKGROUND

LLC resonant converters are widely used in DC-DC converters. The LLCresonant converter is a resonant circuit that adjusts an output voltageby controlling a switching frequency (frequency regulation).

FIG. 1 is a schematic circuit diagram of a conventional LLC resonantconverter, including a switching network, an LLC resonant unit, aresonant transformer, and a rectifier network. The LLC resonant unitincludes a resonant inductor Lr, a resonant capacitor Cr, where Lr maybe a physical inductor device or leakage inductance of a transformer,and an Lm that may be a physical inductor device or an excitationinductance of a transformer. A switching frequency of the switchingnetwork is controlled to regulate an output voltage. An advantage of theLLC resonant converter is implementation of Zero Voltage Switch (ZVS)for a primary-side switching transistor and Zero Current Switch (ZCS)for a secondary-side rectifier diode. By using a soft switch technology,a switching loss can be reduced, thereby improving conversion efficiencyand power density.

FIG. 2 shows output voltage gain curves of a conventional LLC resonantconverter. The horizontal axis denotes working frequencies, and thevertical axis denotes output voltage gains of the LLC resonantconverter. In FIG. 2, different curves represent output voltage gaincurves corresponding to different load ratios of the LLC resonantconverter. According to the gain curves, a working frequency of the LLCresonant converter is much higher than its resonant frequency in a caseof a relatively low output voltage gain. In this case, a switching lossof a primary-side switching transistor increases greatly, and zerocurrent switch is impossible for a secondary-side rectifier diode.Therefore, a switching loss of the LLC resonant convert increasesgreatly, resulting in a significant drop in conversion efficiency of theLLC resonant converter, and further resulting in a drop in load capacityof the LLC resonant converter.

SUMMARY

In view of the foregoing content, a resonant converter circuit and aresonant converter circuit control method in the present application areproposed, so that conversion efficiency of a resonant converter circuitdoes not drop in the case of a low-gain output of the resonant convertercircuit.

According to a first aspect, this application provides a resonantconverter circuit, which is applied to a three-phase circuit. Theresonant converter circuit includes three multi-level inverter circuits,three resonant units, a three-phase transformer, a three-phase rectifiercircuit, an output filter circuit, and a controller. Input terminals ofthe three multi-level inverter circuits are connected in parallel to twoends of an input power supply; input terminals of the three resonantunits are connected to output terminals of the three multi-levelinverter circuits respectively in a one-to-one mode, where the resonantunits are configured to perform voltage conversion on square wavevoltage signals; three primary-side windings of the three-phasetransformer are connected to output terminals of the three resonantunits respectively in a one-to-one mode, where the three-phasetransformer is configured to perform voltage conversion on voltagesignals output by the three resonant units; input terminals of thethree-phase rectifier circuit are connected to secondary-side windingsof the three-phase transformer respectively in a one-to-one mode, wherethe three-phase rectifier circuit is configured to rectify a voltagesignal output by the transformer; an input terminal of the output filtercircuit is connected to output terminals of the three-phase rectifiercircuit, where the output filter circuit is configured to perform wavefiltering on a voltage signal output by the three-phase rectifiercircuit, to obtain an output voltage of the resonant converter circuit;and the controller is configured to, when a required output voltage isless than a preset voltage, control switching statuses of switchingtransistors in the multi-level inverter circuit to reduce an amplitudeof a square wave voltage signal output by the multi-level invertercircuit, so that the resonant converter circuit operates within a presetrange of a resonant frequency.

The resonant converter circuit provided in this embodiment adjusts thefinal output voltage by adjusting input voltages of the resonant units,with no need to substantially adjust a switching frequency of theresonant converter circuit. Therefore, when the output voltage isrelatively low, the resonant converter circuit can still work near itsresonant frequency, thereby reducing a semiconductor switching lossunder low-voltage output, and improving conversion efficiency and loadcapacity of the resonant converter circuit under low-voltage output.

In an implementation of the first aspect, the multi-level invertercircuit is a three-level converter circuit or a five-level convertercircuit.

In an implementation of the first aspect, the three-level convertercircuit includes a bleeder circuit and a first three-level bridge arm.The bleeder circuit includes a first capacitor and a second capacitor,where the first capacitor and the second capacitor are connected inseries to the two ends of the input power supply, a connection pointbetween the first capacitor and the second capacitor is a bleedercircuit middle point, and the bleeder circuit middle point is connectedto a ground terminal; the first three-level bridge arm includes a firstswitching transistor, a second switching transistor, a third switchingtransistor, and a fourth switching transistor; the first switchingtransistor and the second switching transistor are connected inco-directional series and then connected in parallel to two ends of thebleeder circuit, where a connection point between the first switchingtransistor and the second switching transistor is a bridge arm middlepoint of the T-shaped three-level bridge arm, and the bridge arm middlepoint is connected to the input terminal of the resonant unit as theoutput terminal of the multi-level inverter circuit; and the thirdswitching transistor and the fourth switching transistor are connectedin reverse series and then connected between the bleeder circuit middlepoint and the bridge arm middle point.

In another implementation of the first aspect, the three-level convertercircuit includes a bleeder circuit and a second three-level bridge arm.The bleeder circuit includes a first capacitor and a second capacitor,where the first capacitor and the second capacitor are connected inseries to the two ends of the input power supply, a connection pointbetween the first capacitor and the second capacitor is a bleedercircuit middle point, and the bleeder circuit middle point is connectedto a ground terminal; the second three-level bridge arm includes a fifthswitching transistor, a sixth switching transistor, a seventh switchingtransistor, and an eighth switching transistor that are sequentiallyconnected in co-directional series, a first diode, and a second diode; afirst terminal of the fifth switching transistor is connected to apositive terminal of the bleeder circuit, a second terminal of theeighth switching transistor is connected to a negative terminal of thebleeder circuit, and a connection point between the sixth switchingtransistor and the seventh switching transistor is a bridge arm middlepoint of the I-shaped three-level bridge arm; an anode of the firstdiode is connected to the bleeder circuit middle point, and a cathode ofthe first diode is connected to a connection point between the fifthswitching transistor and the sixth switching transistor; and an anode ofthe second diode is connected to a connection point between the seventhswitching transistor and the eighth switching transistor, and a cathodeof the second diode is connected to the bleeder circuit middle point.

In still another implementation of the first aspect, the three-levelconverter circuit is a capacitor-clamped three-level bridge arm. Thecapacitor-clamped three-level bridge arm includes a ninth switchingtransistor, a tenth switching transistor, an eleventh switchingtransistor, a twelfth switching transistor, and a first clampingcapacitor; the ninth switching transistor, the tenth switchingtransistor, the eleventh switching transistor, and the twelfth switchingtransistor are sequentially connected in co-directional series and thenconnected in parallel to the two ends of the input power supply; and apositive electrode of the first clamping capacitor is connected to aconnection point between the ninth switching transistor and the tenthswitching transistor, a negative electrode of the first clampingcapacitor is connected to a connection point between the eleventhswitching transistor and the twelfth switching transistor, and an endvoltage of the first clamping capacitor is E/2, where E is a voltage ofthe input power supply.

In the resonant converter circuit provided in this embodiment, themulti-level inverter circuit is implemented by capacitor-clampedmulti-level bridge arms. Each multi-level bridge arm obtains electricitydirectly from the input power supply, with no need of an extra bleedercircuit formed by two bleeder capacitors. Therefore, for this type ofresonant converter circuit, there is no voltage balance problem betweenbleeder capacitors, and control is easier.

In an implementation of the first aspect, that the controller isconfigured to, when a required output voltage is less than a presetvoltage, control switching statuses of switching transistors in themulti-level inverter circuit is: when the required output voltage isless than a first preset voltage, controlling the three-level convertercircuit to output a square wave voltage signal with a level differenceof E/2, where E is the voltage of the input power supply.

In another implementation of the first aspect, the five-level convertercircuit includes a bleeder circuit and a first five-level bridge arm.The bleeder circuit includes a first capacitor and a second capacitor,where the first capacitor and the second capacitor are connected inseries to the two ends of the input power supply, a connection pointbetween the first capacitor and the second capacitor is a bleedercircuit middle point, and the bleeder circuit middle point is connectedto a ground terminal; the first five-level bridge arm includes a secondclamping capacitor, a thirteenth switching transistor, a fourteenthswitching transistor, a fifteenth switching transistor, a sixteenthswitching transistor, a seventeenth switching transistor, an eighteenthswitching transistor, a nineteenth switching transistor, and a twentiethswitching transistor; the thirteenth switching transistor, thefourteenth switching transistor, the fifteenth switching transistor, andthe sixteenth switching transistor are sequentially connected inco-directional series to obtain a vertical bridge, where the verticalbridge is connected in parallel to two ends of the bleeder circuit, anda connection point between the fourteenth switching transistor and thefifteenth switching transistor is a bridge arm middle point of thefive-level bridge arm; the seventeenth switching transistor and theeighteenth switching transistor are connected in reverse series toobtain a first horizontal bridge, where the first horizontal bridgebridges the bleeder circuit middle point and a connection point betweenthe thirteenth switching transistor and the fourteenth switchingtransistor; the nineteenth switching transistor and the twentiethswitching transistor are connected in reverse series to obtain a secondhorizontal bridge, where the second horizontal bridge bridges thebleeder circuit middle point and a connection point between thefifteenth switching transistor and the sixteenth switching transistor;and the second clamping capacitor spans to connect two ends of thefourteenth switching transistor and the fifteenth switching transistor,and a voltage of the second clamping capacitor is E/4, where E is avoltage of the input power supply.

In still another implementation of the first aspect, the five-levelconverter circuit includes a bleeder circuit and a second five-levelbridge arm. The bleeder circuit includes a first capacitor and a secondcapacitor, where the first capacitor and the second capacitor areconnected in series to the two ends of the input power supply, aconnection point between the first capacitor and the second capacitor isa bleeder circuit middle point, and the bleeder circuit middle point isconnected to a ground terminal; the second five-level bridge armincludes a third clamping capacitor, a twenty-first switchingtransistor, a twenty-second switching transistor, a twenty-thirdswitching transistor, a twenty-fourth switching transistor, atwenty-fifth switching transistor, a twenty-sixth switching transistor,a twenty-seventh switching transistor, and a twenty-eighth switchingtransistor; the twenty-first switching transistor, the twenty-secondswitching transistor, the twenty-third switching transistor, and thetwenty-fourth switching transistor are sequentially connected inco-directional series and then connected in parallel to two ends of thebleeder circuit; the twenty-fifth switching transistor, the twenty-sixthswitching transistor, the twenty-seventh switching transistor, and thetwenty-eighth switching transistor are sequentially connected inco-directional series and then connected in parallel to two ends of thetwenty-second switching transistor and the twenty-third switchingtransistor; and the third clamping capacitor bridges two ends of thetwenty-sixth switching transistor and the twenty-seventh switchingtransistor, and a voltage of the third clamping capacitor is E/4, whereE is a voltage of the input power supply.

In yet another implementation of the first aspect, the preset voltageincludes a second preset voltage, a third preset voltage, and a fourthpreset voltage of which a latter one is greater than a former one, andthe fourth preset voltage is less than the voltage E of the input powersupply; and that the controller is configured to, when a required outputvoltage is less than a preset voltage, control switching statuses ofswitching transistors in the multi-level inverter circuit is: when therequired output voltage is greater than or equal to the third presetvoltage and less than the fourth preset voltage, controlling thefive-level converter circuit to output a square wave voltage signal witha level difference of 3E/4; when the required output voltage is greaterthan or equal to the second preset voltage and less than the thirdpreset voltage, controlling the five-level converter circuit to output asquare wave voltage signal with a level difference of E/2; and when therequired output voltage is less than or equal to the second presetvoltage, controlling the five-level converter circuit to output a squarewave voltage signal with a level difference of E/4.

In the resonant converter circuit provided in this embodiment, themulti-level inverter circuit is a five-level converter circuit. Thefive-level converter circuit can output four input voltages whose leveldifferences are E, 3E/4, E/2, and E/4 respectively, to the resonantunit. The resonant converter circuit can work in different working modesrespectively for four different output voltage requirements. Workingmode control is more refined. This enables the resonant convertercircuit to still work near its resonant frequency when an output voltageis lower, which in turn increases an output voltage range of theresonant converter circuit and its load capacity under low-voltageoutput.

According to a second aspect, this application provides a resonantconverter circuit, which is applied to a single-phase circuit andincludes a multi-level inverter circuit, a resonant unit, a transformer,a rectifier circuit, an output filter circuit, and a controller. Aninput terminal of the multi-level inverter circuit is connected inparallel to two ends of an input power supply, and is configured toconvert a voltage of the input power supply into a square wave voltagesignal, where an amplitude of the square wave voltage signal is lessthan or equal to an amplitude of the input power supply; an inputterminal of the resonant unit is connected to an output terminal of themulti-level inverter circuit, where the resonant unit is configured toperform voltage conversion on the square wave voltage signal; aprimary-side winding of the transformer is connected to an outputterminal of the resonant unit, where the transformer is configured toperform voltage conversion on a voltage signal output by the resonantunit; an input terminal of the rectifier circuit is connected to asecondary-side winding of the transformer, where the rectifier circuitis configured to rectify a voltage signal output by the transformer; aninput terminal of the output filter circuit is connected to an outputterminal of the rectifier circuit, where the output filter circuit isconfigured to perform wave filtering on a voltage signal output by therectifier circuit, to obtain an output voltage of the resonant convertercircuit; and the controller is configured to, when a required outputvoltage is less than a preset voltage, control switching statuses ofswitching transistors in the multi-level inverter circuit to reduce alevel difference of the square wave voltage signal output by themulti-level inverter circuit, so that the resonant converter circuitoperates within a preset range of a resonant frequency.

According to a third aspect, this application further provides aresonant converter circuit system, including at least two resonantconverter circuits described in the first aspect, or at least tworesonant converter circuits described in the second aspect, where the atleast two resonant converter circuits share one output filter circuitand one controller. Input terminals of the at least two resonantconverter circuits are connected in parallel to two ends of an inputpower supply; and output terminals of the at least two resonantconverter circuits are connected in parallel or connected in series, orsome of the output terminals are connected in parallel and the restoutput terminals are connected in series.

In the resonant converter circuit system provided in this embodiment,the output terminals of the at least two resonant converter circuitsare, depending on a power requirement, connected in parallel, connectedin series, or connected in series and then in parallel, so that outputpower of the entire system reaches an application requirement, therebyexpanding an application scope of the resonant converter circuits.

In an implementation of the third aspect, the at least two resonantconverter circuits share one bleeder circuit that is connected inparallel to the two ends of the input power supply, and the bleedercircuit includes a first capacitor and a second capacitor that areconnected in series; and when the resonant converter circuit systemrequires only the first capacitor or the second capacitor to provideenergy, the controller is configured to: control one half of theresonant converter circuits to work in a first preset mode in which thefirst capacitor provides energy, and control the other half of theresonant converter circuits to work in a second preset mode in which thesecond capacitor provides energy.

In the resonant conversion circuit system provided in this embodiment,working modes of different resonant converter circuits are controlled toachieve a voltage balance between the first capacitor and the secondcapacitor in the bleeder circuit, so that the entire resonant conversioncircuit system works properly.

According to a fourth aspect, an embodiment provides a resonantconverter circuit control method, which is applied to the resonantconverter circuit described in the first aspect or the second aspect, orthe resonant converter circuit system described in the third aspect. Themethod includes: controlling, when a required output voltage is lessthan a preset voltage, switching statuses of switching transistors in amulti-level inverter circuit to reduce a level difference of a squarewave voltage signal output by the multi-level inverter circuit, so thatthe resonant converter circuit operates within a preset range of aresonant frequency.

In an implementation of the fourth aspect, the multi-level invertercircuit is a three-level converter circuit; and the controlling, when arequired output voltage is less than a preset voltage, of switchingstatuses of switching transistors in a multi-level inverter circuit toreduce a level difference of a square wave voltage signal output by themulti-level inverter circuit includes: controlling, when the requiredoutput voltage is less than a first preset voltage, the three-levelconverter circuit to output a square wave voltage signal with a leveldifference of E/2, where E is a voltage of an input power supply; andcontrolling, when the required output voltage is greater than or equalto a first preset voltage, the three-level converter circuit to output asquare wave voltage signal with a level difference of E.

In another implementation of the fourth aspect, the multi-level invertercircuit is a five-level converter circuit, the preset voltage includes asecond preset voltage, a third preset voltage, and a fourth presetvoltage of which a latter one is greater than a former one, and thefourth preset voltage is less than a voltage E of an input power supply;and the controlling of, when a required output voltage is less than apreset voltage, switching statuses of switching transistors in amulti-level inverter circuit to reduce a level difference of a squarewave voltage signal output by the multi-level inverter circuit includes:controlling, when the required output voltage is greater than or equalto the fourth preset voltage, the five-level converter circuit to outputa square wave voltage signal with a level difference of E; controlling,when the required output voltage is greater than or equal to the thirdpreset voltage and less than the fourth preset voltage, the five-levelconverter circuit to output a square wave voltage signal with a leveldifference of 3E/4; controlling, when the required output voltage isgreater than or equal to the second preset voltage and less than thethird preset voltage, the five-level converter circuit to output asquare wave voltage signal with a level difference of E/2; andcontrolling, when the required output voltage is less than or equal tothe second preset voltage, the five-level converter circuit to output asquare wave voltage signal with a level difference of E/4.

In still another implementation of the fourth aspect, there are two ormore resonant converter circuits, the resonant converter circuits shareone bleeder circuit that is connected in parallel to two ends of aninput power supply, and the bleeder circuit includes a first capacitorand a second capacitor that are connected in series; and thecontrolling, when a required output voltage is less than a presetvoltage, switching statuses of switching transistors in a multi-levelinverter circuit to reduce a level difference of a square wave voltagesignal output by the multi-level inverter circuit includes: when onlythe first capacitor or the second capacitor is needed to provide energy,controlling one half of the resonant converter circuits to work in afirst preset mode in which only the first capacitor provides energy, andcontrolling the other half of the resonant converter circuits to work ina second preset mode in which only the second capacitor provides energy.

In the resonant converter circuit provided in this embodiment, themulti-level inverter circuit is placed before the resonant unit, and themulti-level inverter circuit can reduce a voltage to be input to theresonant unit. The reduced input voltage of the resonant unit results ina drop in an output voltage of the entire resonant converter circuit. Inthis process, the final output voltage is adjusted by adjusting theinput voltage of the resonant unit, with no need to substantially adjusta switching frequency of the resonant converter circuit. Therefore, whenthe output voltage is relatively low, the resonant converter circuit canstill work near its resonant frequency, thereby reducing a semiconductorswitching loss under low-voltage output, and improving conversionefficiency and load capacity of the resonant converter circuit underlow-voltage output.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic circuit diagram of a conventional LLC resonantconverter;

FIG. 2 is an output voltage gain curve diagram of an LLC resonantconverter;

FIG. 3 is a principle block diagram of a resonant converter circuitaccording to an embodiment of this application;

FIG. 4 is a schematic principle diagram of a resonant converter circuitaccording to an embodiment of this application;

FIG. 5 is a schematic diagram of waveforms at corresponding key pointswhen the resonant converter circuit shown in FIG. 4 is in a working mode1;

FIG. 6 is a schematic diagram of waveforms at corresponding key pointswhen the resonant converter circuit shown in FIG. 4 is in a manner A ofa working mode 2;

FIG. 7 is a schematic diagram of waveforms at corresponding key pointswhen the resonant converter circuit shown in FIG. 4 is in a manner B ofa working mode 2;

FIG. 8 is a schematic principle diagram of another resonant convertercircuit according to an embodiment of this application;

FIG. 9 is a schematic circuit diagram of a three-level bridge armapplied in a resonant converter circuit according to an embodiment ofthis application;

FIG. 10 is a schematic circuit diagram of still another three-levelbridge arm applied in a resonant converter circuit according to anembodiment of this application;

FIG. 11 is a principle block diagram of still another resonant convertercircuit according to an embodiment of this application;

FIG. 12 is a schematic circuit diagram of a capacitor-clampedthree-level bridge arm applied in the resonant converter circuit shownin FIG. 11;

FIG. 13 is a schematic circuit diagram of a five-level bridge armapplied in a resonant converter circuit according to an embodiment ofthis application;

FIG. 14 is a schematic circuit diagram of another five-level bridge armapplied in a resonant converter circuit according to an embodiment ofthis application;

FIG. 15a is a schematic diagram of a square wave voltage signal outputat a bridge arm middle point when a five-level bridge arm is in aworking mode 1;

FIG. 15b is a schematic diagram of a square wave voltage signal outputat a bridge arm middle point when a five-level bridge arm is in aworking mode 3;

FIG. 15c is a schematic diagram of a square wave voltage signal outputat a bridge arm middle point when a five-level bridge arm is in aworking mode 2;

FIG. 15d is a schematic diagram of a square wave voltage signal outputat a bridge arm middle point when another five-level bridge arm is in aworking mode 2;

FIG. 15e is a schematic diagram of a square wave voltage signal outputat a bridge arm middle point when a five-level bridge arm is in aworking mode 4;

FIG. 15f is a schematic diagram of a square wave voltage signal outputat a bridge arm middle point when another five-level bridge arm is in aworking mode 4;

FIG. 16a is a schematic diagram of a connection between primary-sidewindings of a transformer and resonant units according to an embodimentof this application;

FIG. 16b is a schematic diagram of another connection betweenprimary-side windings of a transformer and resonant units according toan embodiment of this application;

FIG. 17 is a schematic connection diagram of a transformer according toan embodiment of this application; and

FIG. 18a and FIG. 18b are principle block diagrams of a power conversionsystem according to an embodiment of this application.

DESCRIPTION OF EMBODIMENTS

For a conventional LLC resonant converter, when an output voltage gainis relatively low, its working frequency is much higher than itsresonant frequency. As a result, the conversion efficiency of the LLCresonant converter is lower, and its load capacity also drops. Thisapplication provides a resonant converter circuit, where a multi-levelinverter circuit is placed before a resonant unit. When a requiredoutput voltage gain is relatively low, an output voltage of themulti-level inverter circuit is decreased to reduce an input voltagethat is input to the resonant unit. The reduced input voltage of theresonant unit results in a drop in an output voltage of the resonantunit, and leads to a drop in an output voltage of the entire LLCresonant converter. However, the switching frequency of the LLC resonantconverter circuit is not changed substantially in this process. In otherwords, the LLC resonant converter circuit may still work near itsresonant frequency. Therefore, the conversion efficiency and loadcapacity of the resonant converter are improved in the case of alow-voltage output.

FIG. 3 is a principle block diagram of a resonant converter circuitaccording to an embodiment of this application. The resonant convertercircuit includes an input power supply, a multi-level inverter circuit110, resonant units 120, a transformer 130, a rectifier circuit 140, andan output filter circuit 150.

A three-phase resonant converter circuit is shown in FIG. 3. Themulti-level inverter circuit 110 includes three multi-level bridge arms;there are three resonant units 120 that are connected to three upstreammulti-level bridge arms respectively in a one-to-one mode; thetransformer 130 is a three-phase transformer, which may be implementedby three independent single-phase transformers; and the rectifiercircuit 140 includes three rectifier bridge arms, each of which may beimplemented by a diode rectifier bridge. The output filter circuit 150may be implemented by a filter capacitor.

Three input terminals of the multi-level inverter circuit 110 areconnected to the input power supply, and three output terminals areconnected to three input terminals of the resonant units 120respectively in a one-to-one mode. Three output terminals of theresonant units 120 are connected to three primary-side windings of thetransformer 130 respectively in a one-to-one mode, and threesecondary-side windings are connected to three input terminals of therectifier circuit 140 respectively in a one-to-one mode; and outputterminals of the rectifier circuit 140 are connected to an inputterminal of the output filter circuit 150, and an output terminal of theoutput filter circuit 150 is connected to a load.

The multi-level inverter circuit 110 is configured to convert a voltagesignal output by the input power supply into a square wave voltagesignal, making a voltage amplitude of the square wave voltage signalless than or equal to a voltage amplitude of the input power supply bycontrolling switching statuses of switching transistors in themulti-level inverter circuit.

A voltage signal output by the resonant unit 120 is input to therectifier circuit 140 after being converted by the transformer 130, andis input to the output filter circuit 150 for wave filtering after beingrectified by the rectifier circuit 140, and finally a steady outputvoltage is obtained.

When an output voltage gain of the LLC resonant converter circuit isneeded to be relatively low, the multi-level inverter circuit 110 iscontrolled to reduce an amplitude of the output voltage to reduce aninput voltage of the resonant unit 120. The reduced input voltage of theresonant unit 120 results in a drop in an output voltage of the resonantunit 120, and leads to a drop in an output voltage of the entire LLCresonant converter.

In an application scenario of a single-phase resonant converter circuit,the multi-level inverter circuit 110 includes one multi-level bridgearm; there is one resonant unit 120; the transformer 130 is onesingle-phase transformer; and the rectifier circuit 140 includes onerectifier bridge arm.

FIG. 4 is a schematic principle diagram of a resonant converter circuitaccording to an embodiment of this application. A three-phase resonantconverter is shown in FIG. 4. This embodiment describes in detailcircuit structures of a multi-level inverter circuit 110, a resonantunit 120, a transformer 130, a rectifier circuit 140, and an outputfilter circuit 150.

As shown in FIG. 4, the multi-level inverter circuit 110 includes ableeder circuit 111 and three multi-level bridge arms 112.

The bleeder circuit 111 includes capacitors C1 and C2 with a samecapacitance. C1 and C2 are connected in series and then connected to twoends of an input power supply, and a middle point between C1 and C2 isconnected to a ground terminal.

The three multi-level bridge arms 112 are T-shaped three-level bridgearms. Each three-level bridge arm includes four switching transistors,where two switching transistors form a vertical bridge, and the othertwo switching transistors form a horizontal bridge.

The switching transistors in this embodiment may be implemented by anyone or more types of switching devices, such as MOS transistors, IGBTtransistors, GaN transistors, and JFET transistors.

In this embodiment, NMOS transistors are used as an example of theswitching devices for description. A drain electrode of the NMOStransistor is a first terminal of the switching transistor, a sourceelectrode is a second terminal of the switching transistor, and a gateelectrode is a control terminal of the switching transistor.

A multi-level bridge arm corresponding to one phase is used as anexample for description. The multi-level bridge arm includes Q1, Q4, Q7,and Q8.

A drain electrode of Q1 is connected to C1 as an input terminal of themulti-level bridge arm, and a source electrode of Q1 is connected to adrain electrode of Q4; and a source electrode of Q4 is connected to C2as another input terminal of the multi-level bridge arm. Q1 and Q4 forma vertical bridge of the multi-level bridge arm, and a connection pointbetween Q1 and Q4 is a bridge arm middle point of the multi-level bridgearm. A drain electrode of Q7 is connected to a middle point between C1and C2, a source electrode of Q7 is connected to a source electrode ofQ8, a drain electrode of Q8 is connected to the connection point betweenQ1 and Q4, and Q7 and Q8 form a horizontal bridge of the multi-levelbridge arm.

A voltage of the input power supply is denoted by E. Because E isequally divided between the capacitors C1 and C2 and the middle pointbetween C1 and C2 is clamped at an electric potential 0, a voltage of C1is E/2, and a voltage of C2 is −E/2. Therefore, three levels E/2, 0, and−E/2 may be obtained at the bridge arm middle point of the T-shapedthree-level bridge arm.

There are three resonant units 120. Each resonant unit includes aresonant inductor Lr, a resonant capacitor Cr, and an excitationinductor Lm, where Lm is excitation inductance of the transformer 130.One terminal of Lr is connected to the bridge arm middle point of themulti-level bridge arm, the other terminal of Lr is connected to oneterminal of Cr, and the other terminal of Cr is connected to aprimary-side winding of the transformer 130.

In this embodiment, the transformer 130 may be three single-phasetransformers. Primary-side windings of the three transformers areconnected to output terminals of the three upstream resonant unitsrespectively. Secondary-side windings of the three single-phasetransformers are connected to middle points of three downstreamrectifier bridge arms respectively in a one-to-one mode.

The rectifier circuit 140 includes three rectifier bridge arms. Eachrectifier bridge arm may be implemented by a half-bridge rectifiercircuit. The output filter circuit 150 may be implemented by a filtercapacitor.

The following uses one phase as an example to describe a workingprinciple of the LLC resonant converter circuit in detail.

Because the multi-level inverter circuit 110 is implemented by athree-level bridge arm, three levels E/2, 0, and −E/2 can be generatedat the bridge arm middle point. Therefore, an amplitude of a square wavevoltage signal output at the bridge arm middle point may be E/2, so thata voltage that is input to the resonant unit drops by half.

In this embodiment, an output voltage gain M=0.5 is used as a criticalpoint to control working modes of the LLC resonant converter circuit.M=0.5 is a typical gain calculated when a transformer turn ratio is 1.When the transformer turn ratio is another value, M is a valuecorresponding to that turn ratio.

(1) Working Mode 1

When the output voltage gain M of the LLC resonant converter circuit isgreater than 0.5, or when an output voltage is greater than a thresholdvoltage, the LLC resonant converter circuit is controlled to work in theworking mode 1.

The output voltage may be calculated based on a formula 1.

Vout=M*Vin/n  (Formula 1)

In the formula 1, Vout is an output voltage of the LLC resonantconverter circuit, Vin is a voltage of an input power supply, M is anoutput voltage gain of the LLC resonant converter circuit, and n is atransformer turn ratio.

A corresponding typical value of the threshold voltage is calculated bygiving M the value 0.5. In actual application, the threshold voltage maybe adjusted near the typical value.

In the working mode 1, the T-shaped three-level bridge arm is controlledto generate a square wave voltage signal with a voltage amplitude E/2(that is, a square wave with amplitudes E/2 and −E/2) at the bridge armmiddle point (that is, a point P1). A level difference of the squarewave voltage signal is E, and therefore the input voltage of theresonant unit is E.

In the working mode 1, drive control logic of the switching transistorsis shown in Table 1.

TABLE 1 Switching transistor state Voltage at the bridge (1 for on and 0for off) arm middle point Working mode Q1 Q4 Q7 Q8 P1 Working mode 1 1 00 0  E/2 0 1 0 0 −E/2

FIG. 5 is a schematic diagram of voltage waveforms at key points whenthe LLC resonant converter circuit is in the working mode 1. As shown inFIGS. 5, Q1 and Q4 are complementarily conducted at approximately a dutycycle of 50%. Q7 and Q8 do not work, and therefore do not affect outputefficiency.

As shown in FIG. 4, when Q1 is on, the voltage at the bridge arm middlepoint P1 is equal to the voltage E/2 of C1; and when Q4 is on, thevoltage at the bridge arm middle point P1 is equal to the voltage −E/2of C2. A waveform of the voltage output at the bridge arm middle pointP1 is a waveform corresponding to P1 shown in FIG. 5. The leveldifference of the square wave voltage signal is E.

(2) Working Mode 2

When M of the LLC resonant converter circuit is less than or equal to0.5, or when an output voltage is less than or equal to a thresholdvoltage, the LLC resonant converter circuit is controlled to work in theworking mode 2.

In this working mode, the T-shaped three-level bridge arm is controlledto generate a square wave voltage signal with amplitudes 0 and E/2 or−E/2 and 0 at a bridge arm middle point P1. A level difference of thesquare wave voltage signal is E/2, and therefore the input voltage ofthe resonant unit is E/2.

Drive control logic of the working mode 2 is shown in Table 2.

Switching transistor state (1 for on, 0 for off, and — Voltage at thebridge for either on or off) arm middle point Working mode Q1 Q4 Q7 Q8P1 Mode 2: 1 0 — 0  E/2 manner A 0 0 1 1 0 Mode 2: 0 1 0 — −E/2 manner B0 0 1 1 0

FIG. 6 is a schematic diagram of waveforms at key points in the manner Aof the working mode 2. Q1 and Q8 are alternately conducted atapproximately a duty cycle of 50%, and Q7 may be always on.

When Q1 is on, the voltage at the bridge arm middle point P1 is equal tothe voltage E/2 of C1; and when Q7 and Q8 are on, voltage drops of Q7and Q8 may be ignored, and the voltage at the bridge arm middle point isequal to a voltage at a bleeder circuit middle point, that is, equal to0. In this working mode, an output voltage at the bridge arm middlepoint is 0 or E/2. Therefore, the input voltage of the resonant unit isE/2.

FIG. 7 is a schematic diagram of waveforms at key points in the manner Bof the working mode 2. Q4 and Q7 are alternately on at approximately aduty cycle of 50%, and Q8 may be always on.

When Q4 is on, the voltage at the bridge arm middle point P1 is equal tothe voltage −E/2 of C2; and when Q7 and Q8 are on, voltage drops of Q7and Q8 may be ignored, and the voltage at the bridge arm middle point isequal to a voltage at a bleeder circuit middle point, that is, equal to0. In this working mode, an output voltage at the bridge arm middlepoint is −E/2 or 0. Therefore, the input voltage of the resonant unit isE/2.

From the working process of the working mode 2, it can be learned thatonly the capacitor C1 provides energy in the manner A, and only thecapacitor C2 provides energy in the manner B. In actual work, the mannerA and the manner B may work alternately to maintain a voltage balancebetween C1 and C2.

The other two phases of the three-phase circuit have the same workingprinciple as the foregoing working process, with a difference being a120° phase difference in one drive control signal corresponding to thethree phases. For example, the same drive control logic is applied toQ1, Q3, and Q5 in three phases A, B, and C, but there is a 120° phasedifference in a control signal corresponding to the three phases.

In the resonant converter circuit provided in this embodiment, athree-level converter circuit is placed before the resonant unit. Thethree-level converter circuit may obtain a square wave voltage signalwith a voltage amplitude E or E/2, and use the square wave voltagesignal as the input voltage of the downstream resonant unit. When arequired output voltage is relatively low, an output voltage of thethree-level converter circuit can be decreased to reduce the inputvoltage of the resonant unit, and finally reduce the output voltage ofthe entire resonant converter circuit. In this process, the final outputvoltage is adjusted by adjusting the input voltage of the resonant unit,with no need to substantially adjust a switching frequency of theresonant converter circuit. Therefore, when the output voltage isrelatively low, the resonant converter circuit can still work near itsresonant frequency, thereby reducing a semiconductor switching lossunder low-voltage output, and improving conversion efficiency and loadcapacity of the resonant converter circuit under low-voltage output.

FIG. 8 is a schematic principle diagram of another resonant convertercircuit according to an embodiment of this application. In thisembodiment, a multi-level bridge arm is implemented by an I-shapedthree-level bridge arm.

As shown in FIG. 8, each I-shaped three-level bridge arm includesswitching transistors Q1, Q2, Q3, and Q4, and diodes D1 and D2. Otherparts are the same as those in the embodiment shown in FIG. 4, anddetails are not described herein again.

The following uses one phase as an example to describe a working processof the resonant converter circuit.

Drive control logic of switching transistors in the circuit shown inFIG. 8 is shown in Table 3.

TABLE 3 Voltage at the Switching transistor state bridge arm OutputWorking (1 for on and 0 for off) middle point gain mode Q1 Q2 Q3 Q4 P1M > 0.5 Working 1 1 0 0  E/2 mode 1 0 0 1 1 −E/2 M ≤ 0.5 Working 1 1 0 0 E/2 mode 2: 0 1 1 0 0 manner A Working 0 1 1 0 0 mode 2: 0 0 1 1 −E/2manner B

In this embodiment, the multi-level inverter circuit is a three-levelconverter circuit. Therefore, M=0.5 is used as a critical point forworking mode switching. M=0.5 is a typical value calculated when atransformer turn ratio is 1. When the turn ratio is another value, thegain value changes accordingly.

1. Working Mode 1

When M>0.5, or when an output voltage is greater than a thresholdvoltage, the LLC resonant converter circuit is controlled to work in theworking mode 1. The threshold voltage may be calculated based on theformula 1.

When Q1 and Q2 are on, the voltage at the bridge arm middle point P1 isequal to a voltage E/2 of C1; and when Q3 and Q4 are on, the voltage atthe bridge arm middle point P1 is equal to a voltage −E/2 of C2. In theworking mode 1, an output voltage at the bridge arm middle point is asquare wave signal with amplitudes −E/2 and E/2. Therefore, an inputvoltage of a resonant unit is a level difference E of the square wavesignal.

2. Working Mode 2

When M<0.5, or when an output voltage is less than or equal to thethreshold voltage, the LLC resonant converter circuit is controlled towork in the working mode 2. The working mode 2 includes the manner A andthe manner B.

(1) Manner A

When Q1 and Q2 are on, the voltage at the bridge arm middle point P1 isequal to a voltage E/2 of C1. When Q1 is off and Q2 and Q3 are on, acurrent on Lr cannot change abruptly, and the current on Lr flows fromleft to right. In this case, D1 and Q2 function as a freewheeling loopof Lr, and make the voltage at the bridge arm middle point P1 equal to0. In this working mode, an output voltage at the bridge arm middlepoint is a square wave signal with amplitudes 0 and E/2. Therefore, aninput voltage of a resonant unit is E/2.

(2) Manner B

When Q2 and Q3 are on, the voltage at the bridge arm middle point P1 isequal to 0. In this state, Q3 and D2 function as a freewheeling loop ofLr. When Q3 and Q4 are on, the voltage at the bridge arm middle point P1is equal to −E/2. That is, the voltage at the bridge arm middle point isa square wave signal with amplitudes 0 and −E/2. Therefore, an inputvoltage of a resonant unit is E/2.

In the resonant converter circuit provided in this embodiment, thethree-level converter circuit is placed before the resonant unit. Thethree-level converter circuit may obtain a square wave voltage signalwith a voltage amplitude E or E/2, and use the square wave voltagesignal as the input voltage of the downstream resonant unit. The finaloutput voltage is adjusted by adjusting the input voltage of theresonant unit, with no need to adjust a switching frequency of theresonant converter circuit. Therefore, when the output voltage isrelatively low, the resonant converter circuit can still work near itsresonant frequency, reducing a semiconductor switching loss, and therebyimproving conversion efficiency and load capacity of the resonantconverter circuit under low-voltage output.

In another embodiment of this application, a three-level bridge arm maybe implemented by a three-level bridge arm circuit shown in FIG. 9. Thethree-level bridge arm includes switching transistors Q1, Q2, Q3, Q4,Q21, and Q24, and diodes D1 and D2.

A difference between the three-level bridge arm shown in FIG. 9 and theI-shaped three-level bridge arm shown in FIG. 8 is that Q21 is connectedin parallel to two ends of a series branch formed by Q1 and Q2, and Q24is connected in parallel to two ends of a series branch formed by Q3 andQ4. Other circuit structures are the same as those of the circuit shownin FIG. 8, and details are not described herein again.

A conduction voltage drop of Q21 is greater than a conduction voltagedrop of the switching transistor Q1 alone, but less than the sum ofconduction voltage drops of Q1 and Q2. Therefore, after Q1 and Q2 areon, controlling Q21 to be on can reduce a conduction loss of thecircuit. Q24 functions the same as Q21.

In still another embodiment of this application, a three-level bridgearm may be implemented by a three-level bridge arm circuit shown in FIG.10. A difference between this three-level bridge arm and the three-levelbridge arm shown in FIG. 9 is that a switching transistor Q31 is used asa substitute for the diode D1, and a switching transistor Q32 is used asa substitute for the diode D2. Q31 and Q32 are always off. In otherwords, Q31 and Q32 do not work. Drive control logic of this three-levelbridge arm is the same as the drive control logic of the three-levelbridge arm shown in FIG. 8, and details are not described herein again.

A conduction voltage drop of Q31 is less than a conduction voltage dropof a diode. Therefore, Q31 functions to reduce a conduction loss. Q32functions the same as Q31.

FIG. 11 is a schematic principle diagram of another resonant convertercircuit according to an embodiment of this application. A differencebetween this embodiment and the embodiment shown in FIG. 4 is that amulti-level inverter circuit in this embodiment is implemented bycapacitor-clamped multi-level bridge arms. Each multi-level bridge armobtains electricity directly from an input power supply, with no need ofan extra bleeder circuit. Therefore, there is no voltage balance problembetween bleeder capacitors.

In this embodiment, the circuits except the multi-level inverter circuitmay be the same as those in FIG. 4, and details are not described hereinagain. This embodiment focuses on a working process of thecapacitor-clamped multi-level bridge arm.

As shown in FIG. 11, the multi-level inverter circuit includes threecapacitor-clamped three-level bridge arms, each including four switchingtransistors and a clamping capacitor. The following uses acapacitor-clamped three-level bridge arm in one phase of circuit as anexample for detailed description.

The capacitor-clamped three-level bridge arm includes switchingtransistors Q1 to Q4, and a clamping capacitor C1. Q1 to Q4 aresequentially connected in series to form a series branch; and a drainelectrode of Q1 is connected to one end of the input power supply as oneend of the series branch, and a source electrode of Q4 is connected tothe other end of the input power supply as the other end of the seriesbranch. A connection point between Q2 and Q3 is a bridge arm middlepoint; and a positive electrode of C1 is connected to a connection pointbetween Q1 and Q2, and a negative electrode of C1 is connected to aconnection point between Q3 and Q4.

A voltage of the input power supply is E, a voltage of the clampingcapacitor C1 is E/2, and the capacitor-clamped three-level bridge armcan output three levels: E, E/2, and 0.

Drive control logic of the capacitor-clamped three-level bridge arm isshown in Table 4.

TABLE 4 Voltage at the Switching transistor state bridge arm OutputWorking (1 for on and 0 for off) middle point gain mode Q1 Q2 Q3 Q4 P1M > 0.5 Working 1 1 0 0 E mode 1 0 0 1 1 0 M ≤ 0.5 Working 1 0 1 0 E/2mode 2 0 1 0 1 0 0 1 1 0

State 1: Q1 and Q2 are on, and the voltage at the bridge arm middlepoint is equal to a voltage at a positive electrode of the input powersupply, that is, E.

State 2: Q3 and Q4 are on, and the voltage at the bridge arm middlepoint is equal to a voltage at a negative electrode of the input powersupply, that is, 0.

State 3: Q1 and Q3 are on. The input power supply charges the clampingcapacitor C1, the voltage of the input power supply is E, and thevoltage of C1 is equal to E/2 in a steady state. Therefore, the voltageat the bridge arm middle point is equal to E/2.

State 4: Q2 and Q4 are on. The clamping capacitor C1 discharges.Therefore, the voltage at the bridge arm middle point is equal to thevoltage of C1, that is, E/2.

The voltages at the bridge arm middle point corresponding to the state 3and the state 4 are both E/2. The clamping capacitor C1 is charged inthe state 3, but discharges in the state 4. In actual application, whichstate is active is controlled based on the voltage of the clampingcapacitor C1.

In this embodiment, the multi-level inverter circuit is a three-levelconverter circuit. Therefore, M=0.5 is used as a critical point forworking mode switching. M=0.5 is a typical value calculated when atransformer turn ratio is 1. When the turn ratio is another value, thegain value changes accordingly.

When M>0.5, the LLC resonant converter circuit is controlled to work inthe working mode 1. In this working mode, the state 1 and the state 2are active alternately, and the voltage at the bridge arm middle pointis a square wave signal with amplitudes 0 and E. Therefore, an inputvoltage of a resonant unit is E.

When M<0.5, the LLC resonant converter circuit is controlled to work inthe working mode 2. In this working mode, the state 3 (or the state 4)and the state 2 are active alternately, where the state 3 and the state4 are active alternately depending on an actual requirement. The voltageat the bridge arm middle point is a square wave signal with amplitudes 0and E/2. Therefore, an input voltage of a resonant unit is E/2.

FIG. 12 is a schematic circuit diagram of another capacitor-clampedthree-level bridge arm according to an embodiment of this application. Adifference between the multi-level bridge arm provided in thisembodiment and the three-level bridge arm shown in FIG. 11 is thatswitching transistors Q21 and Q24 are added.

Q21 bridges two ends of a series branch formed by Q1 and Q2, and Q24bridges two ends of a series branch formed by Q3 and Q4. Q21 and Q24function to reduce a conduction voltage drop.

A switching status of the capacitor-clamped three-level bridge arm isthe same as the switching status of the three-level bridge arm shown inFIG. 9, and details are not described herein again.

In a resonant converter circuit provided in this embodiment, amulti-level inverter circuit is implemented by capacitor-clampedmulti-level bridge arms. Each multi-level bridge arm obtains electricitydirectly from an input power supply, with no need of an extra bleedercircuit formed by two bleeder capacitors. Therefore, for this type ofresonant converter circuit, there is no voltage balance problem betweenbleeder capacitors, and control is easier.

In still another embodiment of this application, a multi-level invertercircuit may be implemented by a five-level converter circuit.

FIG. 13 is a schematic circuit diagram of a five-level bridge armapplied in a resonant converter circuit. A difference between thisembodiment and the embodiment shown in FIG. 4 is that the three-levelbridge arms in FIG. 4 are replaced by five-level bridge arms, whileother parts are the same as those in FIG. 4. This embodiment focuses onwork of a five-level converter circuit and a working mode switchingprocess of the LLC resonant converter circuit.

As shown in FIG. 13, the five-level bridge arm includes switchingtransistors Q1 to Q8, and a clamping capacitor C11.

A series branch resulting from sequentially connecting Q1, Q2, Q3, andQ4 in series is connected to two ends of a bleeder circuit. A drainelectrode of Q1 is connected to C1, a source electrode of Q1 isconnected to a drain electrode of Q2, a source electrode of Q2 isconnected to a drain electrode of Q3, a source electrode of Q3 isconnected to a drain electrode of Q4, and a source electrode of Q4 isconnected to C2. A connection point between Q2 and Q3 is a bridge armmiddle point.

C11 is connected in parallel to two ends of a connection branch formedby Q2 and Q3; and when C11 is in a steady state, voltages of the twoends are equal to E/4.

Q5 and Q6 bridge between a bleeder circuit middle point O and aconnection point between Q1 and Q2. A drain electrode of Q5 is connectedto the bleeder circuit middle point, a source electrode of Q5 isconnected to a source electrode of Q6, and a drain electrode of Q6 isconnected to the connection point between Q1 and Q2.

Q7 and Q8 bridge between the bleeder circuit middle point O and aconnection point between Q3 and Q4. Connection manners of Q7 and Q8 arethe same as those of Q5 and Q6, and details are not described hereinagain.

Signals of five levels E/2, E/4, 0, −E/4, and −E/2 can be obtained atthe bridge arm middle point of the five-level bridge arm. Drive controllogic of the five-level bridge arm is shown in the following table.

TABLE 5 Switching transistor state (1 for on, 0 for off, and — foreither on or off) Output level at the bridge Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 armmiddle point 1 1 0 0 — 0 — 0  E/2 1 0 1 0 — 0 — 0  E/4 0 1 0 0 — 0 1 1 00 1 0 — 0 1 1 0 0 1 0 0 1 1 0 — 0 0 1 0 1 1 0 — −E/4 0 1 0 1 0 — 0 — 0 01 1 0 — 0 — −E/2

The following describes various states of the five-level bridge arm oneby one in combination with the foregoing table.

State 1: Q1 and Q2 are on. A voltage at the bridge arm middle point P1is equal to a voltage of C1, that is, E/2.

State 2: Q1 and Q3 are on. C1 charges the clamping capacitor C11 throughQ1 and Q3. The voltage at the bridge arm middle point is equal to thevoltage of C1 minus a voltage of C11, that is, E/2−E/4=E/4.

State 3: Q2, Q5, Q7, and Q8 are on. Q7 and Q8 being on make a voltage ata negative electrode of C11 equal to 0. When C11 is in a steady state,the voltage of C11 is equal to E/4. Therefore, a voltage at a positiveelectrode of C11 is E/4. In this state, C11 discharges, so that thevoltage at the bridge arm middle point P1 is equal to the voltage E/4 ofC11.

State 4: Q3, Q7, and Q8 are on. The voltage at the bridge arm middlepoint is equal to a voltage at the bleeder circuit middle point, thatis, 0.

State 5: Q2, Q5, and Q6 are on. The voltage at the bridge arm middlepoint is equal to the voltage at the bleeder circuit middle point, thatis, 0.

State 6: Q3, Q5, and Q6 are on. In this state, the voltage at thepositive electrode of C11 is equal to 0, and the voltage at the negativeelectrode of C11 is equal to −E/4. Q3 being on makes the voltage at thebridge arm middle point P1 equal to the voltage at the negativeelectrode of C11, that is, −E/4.

State 7: Q2 and Q4 are on. In this state, Q4 being on makes the voltageat the negative electrode of C11 equal to −E/2, and makes the voltage atthe positive electrode of C11 equal to −E/2+E/4=−E/4. Q2 being on makesthe voltage at the bridge arm middle point equal to the voltage at thepositive electrode of C11, that is, E/4.

State 8: Q3 and Q4 are on. The voltage at the bridge arm middle point P1is equal to a voltage at a negative electrode of C2, that is, −E/2.

FIG. 14 is a schematic circuit diagram of another five-level bridge armapplied in a resonant converter circuit according to an embodiment ofthis application.

As shown in FIG. 14, the five-level bridge arm includes switchingtransistors Q1 to Q8, and a capacitor C11. Q1 to Q4 are sequentiallyconnected in series to form a first series branch, and Q5 to Q8 aresequentially connected in series to form a second series branch.

A drain electrode of Q1 is connected to a positive electrode of C1 asone end of the first series branch, a source electrode of Q4 isconnected to a negative electrode of C2 as the other end of the firstseries branch, and a connection point between Q2 and Q3 is connected toa connection point between C1 and C2. A drain electrode of Q5 isconnected to a connection point between Q1 and Q2 as one end of thesecond series branch, a source electrode of Q8 is connected to aconnection point between Q3 and Q4 as the other end of the second seriesbranch, and a connection point between Q6 and Q7 is a bridge arm middlepoint P1.

A positive electrode of C11 is connected to a connection point betweenQ5 and Q6, a negative electrode of C11 is connected to a connectionpoint between Q7 and Q8, and a voltage of C11 is equal to E/4.

Signals of five levels E/2, E/4, 0, −E/4, and −E/2 can be obtained atthe bridge arm middle point of the five-level bridge arm. Drive controllogic of the five-level bridge arm is shown in the following table.

TABLE 6 Switching transistor state (1 for on, 0 for off, and — foreither on or off) Output level at the bridge Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 armmiddle point 1 0 — 0 1 1 0 0  E/2 1 0 — 0 1 0 1 0  E/4 — 0 1 0 0 1 0 1 —0 1 0 0 0 1 1 0 0 1 0 — 1 1 0 0 0 1 0 — 1 0 1 0 −E/4 0 — 0 1 0 1 0 1 0 —0 1 0 0 1 1 −E/2

State 1: Q1, Q5, and Q6 are on. A voltage at the bridge arm middle pointis equal to a voltage of C1, that is, E/2.

State 2: Q1, Q5, and Q7 are on. C11 is charged. The voltage at thebridge arm middle point is equal to the voltage of C1 minus a voltage ofC11, that is, E/2−E/4=E/4.

State 3: Q3, Q6, and Q8 are on. C11 discharges. The voltage at thebridge arm middle point is equal to the voltage of C11, that is, E/4.

State 4: Q3, Q7, and Q8 are on. The voltage at the bridge arm middlepoint is equal to a voltage at a bleeder circuit middle point, that is,0.

State 5: Q2, Q5, and Q6 are on. The voltage at the bridge arm middlepoint is equal to the voltage at the bleeder circuit middle point, thatis, 0.

State 6: Q2, Q5, and Q7 are on. In this state, a voltage at the positiveelectrode of C11 is equal to 0, a voltage at the negative electrode ofC11 is equal to −E/4, and the voltage at the bridge arm middle point isequal to the voltage at the negative electrode of C11, that is, E/4.

State 7: Q4, Q6, and Q8 are on. In this state, the voltage at thenegative electrode of C11 is equal to −E/2, and the voltage at thebridge arm middle point is equal to −E/2 plus the voltage of C11, thatis, −E/2+E/4=−E/4.

State 8: Q4, Q7, and Q8 are on. In this state, the voltage at the bridgearm middle point is equal to a voltage at the negative electrode of C2,that is, −E/2.

Because the five-level bridge arm can output five different amplitudesof level at the bridge arm middle point, 10 square wave voltages ofdifferent amplitudes may be generated at an output terminal of themulti-level inverter circuit by combining any two levels, as shown inthe following table.

TABLE 7 Amplitude of a square wave signal at the output Working modeterminal of the multi-level inverter circuit Working mode 1 (E/2, −E/2)Working mode 3 (E/2, −E/4) (E/4, −E/2) Working mode 2 (E/2, 0)    (E/4,−E/4) (0, −E/2) Working mode 4 (E/2, E/4)  (E/4, 0)    (0, −E/4) (−E/4,−E/2)

In the working mode 1, a level difference of a square wave voltageoutput by the multi-level inverter circuit is E; in the working mode 2,a level difference of a square wave voltage output by the multi-levelinverter circuit is 3E/4; in the working mode 3, a level difference of asquare wave voltage output by the multi-level inverter circuit is E/2;and in the working mode 4, a level difference of a square wave voltageoutput by the multi-level inverter circuit is E/4. Therefore, there arethree critical points for working mode switching: M=0.75, 0.5, and 0.25.These three values may be typical values calculated when a transformerturn ratio is 1.

When M>0.75, or when an output voltage is greater than a first presetvoltage, the LLC resonant converter circuit is controlled to work in theworking mode 1. In the working mode 1, a waveform of a square wavevoltage signal output by the multi-level inverter circuit is shown inFIG. 15 a.

When 0.75≥M>0.5, or when an output voltage is greater than a secondpreset voltage and less than the first preset voltage, the LLC resonantconverter circuit is controlled to work in the working mode 3. In theworking mode 3, a waveform of a square wave voltage signal output by themulti-level inverter circuit is shown in FIG. 15 b.

When 0.5≥M>0.25, or when an output voltage is greater than a thirdpreset voltage and less than the second preset voltage, the LLC resonantconverter circuit is controlled to work in the working mode 2. In theworking mode 2, a waveform of a square wave voltage signal output by themulti-level inverter circuit is shown in FIG. 15c and FIG. 15 d.

When M<0.25, or when an output voltage is less than the third presetvoltage, the LLC resonant converter circuit is controlled to work in theworking mode 4. In the working mode 4, a waveform of a square wavevoltage signal output by the multi-level inverter circuit is shown inFIG. 15 e.

In the resonant converter circuit provided in this embodiment, afive-level converter circuit is placed before a resonant unit. When arequired output voltage gain is relatively low, the five-level convertercircuit may be controlled to reduce an input voltage to be input to theresonant unit. The five-level converter circuit can output four inputvoltages whose level differences are E, 3E/4, E/2, and E/4 respectively,to the resonant unit. The resonant converter circuit can work indifferent working modes respectively for four different output voltagerequirements. Working mode control is more refined. This enables theresonant converter circuit to still work near its resonant frequencywhen the output voltage is lower, which in turn increases an outputvoltage range of the resonant converter circuit and its load capacityunder low-voltage output.

In another aspect, in the foregoing resonant converter circuitembodiments, the three primary-side windings of the transformer assume atriangle connection style or a star connection style, and the threesecondary-side windings may assume a triangle connection style or a starconnection style. The triangle connection style means that threewindings are connected in a head-to-tail manner to form a triangle, andthree lead-out wires are led out from three connection points; and thestart connection style means that tails of three windings are connectedto form a common point, referred to as a neutral point, and threelead-out wires are led out from heads of the three windingsrespectively.

The resonant inductor, the resonant capacitor, and the primary-sidewindings of the transformer are connected in the following two manners:

(1) As shown in FIG. 16a , in a resonant unit, a resonant inductor Lr isleakage inductance of a transformer, an excitation inductor Lm isexcitation inductance of the transformer, and primary-side windings ofthe transformer are connected to form a triangle.

In another embodiment of this application, a resonant inductor Lr isleakage inductance of a transformer, an excitation inductor Lm isexcitation inductance of the transformer, and primary-side windings ofthe transformer are connected to form a star structure.

(2) As shown in FIG. 16b , a resonant capacitor Cr in a resonant unitand primary-side windings are connected in series, an excitationinductor Lm is excitation inductance of a transformer, Lr is astandalone inductor device, and the primary-side windings of thetransformer are connected to form a triangle. In another embodiment ofthis application, the primary-side windings in FIG. 16b mayalternatively be connected to form a star structure.

In still another aspect, when the resonant converter circuit provided inthe embodiments of this application is applied in a three-phase circuit,a three-phase transformer may be implemented by connecting twotransformers in series.

As shown in FIG. 17, a non-dotted terminal of a primary-side winding ofa transformer T1 is connected to a dotted terminal of a primary-sidewinding of a transformer T2, and a resulting connection point isconnected to an output terminal of a multi-level bridge arm; and adotted terminal of the primary-side winding of T1 and a non-dottedterminal of the primary-side winding of T2 are connected to outputterminals of the other two multi-level bridge arms respectively. Anon-dotted terminal of a secondary-side winding of T1 is connected to adotted terminal of a secondary-side winding of T2, and an obtainedconnection point is connected to an input terminal of a rectifiercircuit; and a dotted terminal of the secondary-side winding of T1 and anon-dotted terminal of the secondary-side winding of T2 are connected tothe other two input terminals of the rectifier circuit respectively.

In yet another aspect, in the foregoing resonant converter circuitembodiments, the rectifier circuit may be implemented by a rectifierhalf bridge that is implemented by MOS transistors. In this case, diodesin the rectifier circuit shown in FIG. 4 are replaced by MOStransistors. In addition, JFET transistors, GaN transistors, or IGBTtransistors may also be used as substitutes for the diodes. Conductionlosses of switching transistors such as MOS transistors, JFETtransistors, GaN transistors, and IGBT transistors are less thanconduction losses of diodes, and therefore a loss of the resonantconverter circuit is further reduced.

In another application scenario, a relatively high power is needed, andone resonant converter circuit provided in the foregoing embodimentscannot meet the power requirement. In this case, a plurality of resonantconverter circuits may be used and connected in series or parallel forimplementation.

FIG. 18a and FIG. 18b is a principle block diagram of a power conversionsystem according to an embodiment of this application. The powerconversion system includes N resonant converter circuits, where the Nresonant converter circuits share one output filter circuit.

Input terminals of the resonant converter circuits are connected inparallel to two ends of an input power supply, and their outputterminals may be connected in parallel or connected in series, or outputterminals of some of the resonant converter circuits are connected inparallel and then the parallelly connected terminals are connected tooutput terminals of the other resonant converter circuits in series.

If the output terminals of the resonant converter circuits are connectedin parallel, an output current of the entire system is equal to a sum ofoutput currents of the resonant converter circuits. This is applicableto application scenarios characterized by low-voltage and high-currentoutput. If the output terminals of the resonant converter circuits areconnected in series, an output voltage of the entire system is equal toa sum of output voltages of the resonant converter circuits. This isapplicable to application scenarios characterized by high-voltage andlow-current output.

When the resonant converter circuits assume the circuit topology shownin FIG. 4, FIG. 8, or FIG. 9, these circuit topologies include a bleedercircuit, and the bleeder circuit is implemented by two bleedercapacitors. Therefore, a voltage balance needs to be achieved betweenthe two bleeder capacitors.

In such application scenarios, when the resonant converter circuits workin the working mode 2, the different resonant converter circuits may becontrolled to work in the manner A and the manner B respectively. Forexample, when N=2, one resonant converter circuit works in the manner Aof the working mode 2, and the other resonant converter circuit works inthe manner B of the working mode 2. In this way, a voltage balance isachieved between the two capacitors in the bleeder circuit, furtherenabling the entire power conversion system to work properly.

In still yet another aspect, this application further provides aresonant converter circuit control method. The method is applied in theforegoing resonant converter circuit embodiments or resonant convertercircuit system embodiment. When a required output voltage of a resonantconverter circuit is less than a preset voltage, switching statuses ofswitching transistors included in a multi-level inverter circuit iscontrolled to reduce a level difference of a square wave voltage signaloutput by the multi-level inverter circuit, so that the correspondingresonant converter circuit operates within a preset range of a resonantfrequency.

In an embodiment of this application, if the multi-level invertercircuit is a three-level converter circuit, when the required outputvoltage is less than a first preset voltage, the three-level convertercircuit is controlled to output a square wave voltage signal with alevel difference of E/2, where E is a voltage of an input power supply;and when the required output voltage is greater than or equal to thefirst preset voltage, the three-level converter circuit is controlled tooutput a square wave voltage signal with a level difference of E.

In another embodiment of this application, if the multi-level invertercircuit is a five-level converter circuit, the preset voltage includes asecond preset voltage, a third preset voltage, and a fourth presetvoltage of which a latter one is greater than a former one, and thefourth preset voltage is less than a voltage E of an input power supply.

When the required output voltage is greater than or equal to the fourthpreset voltage, the five-level converter circuit is controlled to outputa square wave voltage signal with a level difference of E.

When the required output voltage is greater than or equal to the thirdpreset voltage and less than the fourth preset voltage, the five-levelconverter circuit is controlled to output a square wave voltage signalwith a level difference of 3E/4.

When the required output voltage is greater than or equal to the secondpreset voltage and less than the third preset voltage, the five-levelconverter circuit is controlled to output a square wave voltage signalwith a level difference of E/2.

When the required output voltage is less than or equal to the secondpreset voltage, the five-level converter circuit is controlled to outputa square wave voltage signal with a level difference of E/4.

In still another embodiment (the embodiment shown in FIG. 18a and FIG.18b ) of this application, there are two or more resonant convertercircuits, the resonant converter circuits share one bleeder circuit thatis connected in parallel to the two ends of the input power supply, andthe bleeder circuit includes a first capacitor and a second capacitorthat are connected in series.

When only the first capacitor (or the second capacitor) is needed toprovide energy (for example, in the working mode 2), one half of theresonant converter circuits are controlled to work in a first presetmode in which only the first capacitor provides energy, and the otherhalf of the resonant converter circuits are controlled to work in asecond preset mode in which only the second capacitor provides energy,so that a voltage balance is achieved between the first capacitor andthe second capacitor in the bleeder circuit.

All or some of the foregoing embodiments may be implemented by software,hardware, firmware, or any combination thereof. When software is used toimplement the embodiments, the embodiments may be implemented completelyor partially in a form of a computer program product. The computerprogram product includes one or more computer instructions. When thecomputer program instructions are loaded and executed on a computer, theprocedure or functions according to the embodiments of the presentapplication are all or partially generated. The computer may be ageneral-purpose computer, a dedicated computer, a computer network, orother programmable apparatuses. The computer instructions may be storedin a computer-readable storage medium or may be transmitted from onecomputer-readable storage medium to another computer-readable storagemedium. For example, the computer instructions may be transmitted from awebsite, computer, server, or data center to another website, computer,server, or data center in a wired (for example, through a coaxial cable,an optical fiber, or a digital subscriber line (DSL)) or wireless (forexample, through infrared, radio, microwave, or the like) manner. Thecomputer-readable storage medium may be any usable medium accessible bya computer, or a data storage device, such as a server or a data center,integrating one or more usable media. The usable medium may be amagnetic medium (for example, a floppy disk, a hard disk, or a magnetictape), an optical medium (for example, a DVD), a semiconductor medium(for example, a Solid State Disk (SSD)), or the like.

It should be noted that the embodiments in this specification aredescribed in a progressive manner, each embodiment focuses on itsdifference from other embodiments. For the same or similar parts in theembodiments, mutual reference may be made.

What is claimed is:
 1. A resonant converter circuit, applied to athree-phase circuit, and comprising three multi-level inverter circuits,three resonant units, a three-phase transformer, a three-phase rectifiercircuit, an output filter circuit, and a controller, wherein inputterminals of the three multi-level inverter circuits are connected inparallel to two ends of an input power supply; input terminals of thethree resonant units are connected to output terminals of the threemulti-level inverter circuits respectively in a one-to-one mode, whereinthe resonant units are configured to perform voltage conversion onsquare wave voltage signals; three primary-side windings of thethree-phase transformer are connected to output terminals of the threeresonant units respectively in a one-to-one mode, wherein thethree-phase transformer is configured to perform voltage conversion onvoltage signals output by the three resonant units; input terminals ofthe three-phase rectifier circuit are connected to secondary-sidewindings of the three-phase transformer respectively in a one-to-onemode, wherein the three-phase rectifier circuit is configured to rectifya voltage signal output by the transformer; an input terminal of theoutput filter circuit is connected to output terminals of thethree-phase rectifier circuit, wherein the output filter circuit isconfigured to perform wave filtering on a voltage signal output by thethree-phase rectifier circuit, to obtain an output voltage of theresonant converter circuit; and the controller is configured to, when arequired output voltage is less than a preset voltage, control switchingstatuses of switching transistors in the multi-level inverter circuit toreduce an amplitude of a square wave voltage signal output by themulti-level inverter circuit, so that the resonant converter circuitoperates within a preset range of a resonant frequency.
 2. The resonantconverter circuit according to claim 1, wherein the multi-level invertercircuit is a three-level converter circuit or a five-level convertercircuit.
 3. The resonant converter circuit according to claim 2, whereinthe three-level converter circuit comprises a bleeder circuit and afirst three-level bridge arm, wherein the bleeder circuit comprises afirst capacitor and a second capacitor, wherein the first capacitor andthe second capacitor are connected in series to the two ends of theinput power supply, a connection point between the first capacitor andthe second capacitor is a bleeder circuit middle point, and the bleedercircuit middle point is connected to a ground terminal; the firstthree-level bridge arm comprises a first switching transistor, a secondswitching transistor, a third switching transistor, and a fourthswitching transistor; the first switching transistor and the secondswitching transistor are connected in co-directional series and thenconnected in parallel to two ends of the bleeder circuit, wherein aconnection point between the first switching transistor and the secondswitching transistor is a bridge arm middle point of the T-shapedthree-level bridge arm, and the bridge arm middle point is connected tothe input terminal of the resonant unit as the output terminal of themulti-level inverter circuit; and the third switching transistor and thefourth switching transistor are connected in reverse series and thenconnected between the bleeder circuit middle point and the bridge armmiddle point.
 4. The resonant converter circuit according to claim 2,wherein the three-level converter circuit comprises a bleeder circuitand a second three-level bridge arm, wherein the bleeder circuitcomprises a first capacitor and a second capacitor, wherein the firstcapacitor and the second capacitor are connected in series to the twoends of the input power supply, a connection point between the firstcapacitor and the second capacitor is a bleeder circuit middle point,and the bleeder circuit middle point is connected to a ground terminal;the second three-level bridge arm comprises a fifth switchingtransistor, a sixth switching transistor, a seventh switchingtransistor, and an eighth switching transistor that are sequentiallyconnected in co-directional series, a first diode, and a second diode; afirst terminal of the fifth switching transistor is connected to apositive terminal of the bleeder circuit, a second terminal of theeighth switching transistor is connected to a negative terminal of thebleeder circuit, and a connection point between the sixth switchingtransistor and the seventh switching transistor is a bridge arm middlepoint of the I-shaped three-level bridge arm; an anode of the firstdiode is connected to the bleeder circuit middle point, and a cathode ofthe first diode is connected to a connection point between the fifthswitching transistor and the sixth switching transistor; and an anode ofthe second diode is connected to a connection point between the seventhswitching transistor and the eighth switching transistor, and a cathodeof the second diode is connected to the bleeder circuit middle point. 5.The resonant converter circuit according to claim 2, wherein thethree-level converter circuit is a capacitor-clamped three-level bridgearm, wherein the capacitor-clamped three-level bridge arm comprises aninth switching transistor, a tenth switching transistor, an eleventhswitching transistor, a twelfth switching transistor, and a firstclamping capacitor; the ninth switching transistor, the tenth switchingtransistor, the eleventh switching transistor, and the twelfth switchingtransistor are sequentially connected in co-directional series and thenconnected in parallel to the two ends of the input power supply; and apositive electrode of the first clamping capacitor is connected to aconnection point between the ninth switching transistor and the tenthswitching transistor, a negative electrode of the first clampingcapacitor is connected to a connection point between the eleventhswitching transistor and the twelfth switching transistor, and an endvoltage of the first clamping capacitor is E/2, wherein E is a voltageof the input power supply.
 6. The resonant converter circuit accordingto claim 2, wherein that the controller is configured to, when arequired output voltage is less than a preset voltage, control switchingstatuses of switching transistors in the multi-level inverter circuitcomprises: when the required output voltage is less than a first presetvoltage, controlling the three-level converter circuit to output asquare wave voltage signal with a level difference of E/2, wherein E isthe voltage of the input power supply.
 7. The resonant converter circuitaccording to claim 2, wherein the five-level converter circuit comprisesa bleeder circuit and a first five-level bridge arm, wherein the bleedercircuit comprises a first capacitor and a second capacitor, wherein thefirst capacitor and the second capacitor are connected in series to thetwo ends of the input power supply, a connection point between the firstcapacitor and the second capacitor is a bleeder circuit middle point,and the bleeder circuit middle point is connected to a ground terminal;the first five-level bridge arm comprises a second clamping capacitor, athirteenth switching transistor, a fourteenth switching transistor, afifteenth switching transistor, a sixteenth switching transistor, aseventeenth switching transistor, an eighteenth switching transistor, anineteenth switching transistor, and a twentieth switching transistor;the thirteenth switching transistor, the fourteenth switchingtransistor, the fifteenth switching transistor, and the sixteenthswitching transistor are sequentially connected in co-directional seriesto obtain a vertical bridge, wherein the vertical bridge is connected inparallel to two ends of the bleeder circuit, and a connection pointbetween the fourteenth switching transistor and the fifteenth switchingtransistor is a bridge arm middle point of the five-level bridge arm;the seventeenth switching transistor and the eighteenth switchingtransistor are connected in reverse series to obtain a first horizontalbridge, wherein the first horizontal bridge bridges the bleeder circuitmiddle point and a connection point between the thirteenth switchingtransistor and the fourteenth switching transistor; the nineteenthswitching transistor and the twentieth switching transistor areconnected in reverse series to obtain a second horizontal bridge,wherein the second horizontal bridge bridges the bleeder circuit middlepoint and a connection point between the fifteenth switching transistorand the sixteenth switching transistor; and the second clampingcapacitor spans to connect two ends of the fourteenth switchingtransistor and the fifteenth switching transistor, and a voltage of thesecond clamping capacitor is E/4, wherein E is a voltage of the inputpower supply.
 8. The resonant converter circuit according to claim 2,wherein the five-level converter circuit comprises a bleeder circuit anda second five-level bridge arm, wherein the bleeder circuit comprises afirst capacitor and a second capacitor, wherein the first capacitor andthe second capacitor are connected in series to the two ends of theinput power supply, a connection point between the first capacitor andthe second capacitor is a bleeder circuit middle point, and the bleedercircuit middle point is connected to a ground terminal; the secondfive-level bridge arm comprises a third clamping capacitor, atwenty-first switching transistor, a twenty-second switching transistor,a twenty-third switching transistor, a twenty-fourth switchingtransistor, a twenty-fifth switching transistor, a twenty-sixthswitching transistor, a twenty-seventh switching transistor, and atwenty-eighth switching transistor; the twenty-first switchingtransistor, the twenty-second switching transistor, the twenty-thirdswitching transistor, and the twenty-fourth switching transistor aresequentially connected in co-directional series and then connected inparallel to two ends of the bleeder circuit; the twenty-fifth switchingtransistor, the twenty-sixth switching transistor, the twenty-seventhswitching transistor, and the twenty-eighth switching transistor aresequentially connected in co-directional series and then connected inparallel to two ends of the twenty-second switching transistor and thetwenty-third switching transistor; and the third clamping capacitorbridges two ends of the twenty-sixth switching transistor and thetwenty-seventh switching transistor, and a voltage of the third clampingcapacitor is E/4, wherein E is a voltage of the input power supply. 9.The resonant converter circuit according to claim 2, wherein the presetvoltage comprises a second preset voltage, a third preset voltage, and afourth preset voltage of which a latter one is greater than a formerone, and the fourth preset voltage is less than the voltage E of theinput power supply; and that the controller is configured to, when arequired output voltage is less than a preset voltage, control switchingstatuses of switching transistors in the multi-level inverter circuitcomprises: when the required output voltage is greater than or equal tothe third preset voltage and less than the fourth preset voltage,controlling the five-level converter circuit to output a square wavevoltage signal with a level difference of 3E/4; when the required outputvoltage is greater than or equal to the second preset voltage and lessthan the third preset voltage, controlling the five-level convertercircuit to output a square wave voltage signal with a level differenceof E/2; and when the required output voltage is less than or equal tothe second preset voltage, controlling the five-level converter circuitto output a square wave voltage signal with a level difference of E/4.10. A resonant converter circuit system, comprising at least tworesonant converter circuits according to claim 1, wherein the at leasttwo resonant converter circuits share one output filter circuit and onecontroller, wherein input terminals of the at least two resonantconverter circuits are connected in parallel to two ends of an inputpower supply; and output terminals of the at least two resonantconverter circuits are connected in parallel or connected in series, orsome of the output terminals are connected in parallel and the restoutput terminals are connected in series.
 11. The resonant convertercircuit system according to claim 10, wherein the at least two resonantconverter circuits share one bleeder circuit that is connected inparallel to the two ends of the input power supply, and the bleedercircuit comprises a first capacitor and a second capacitor that areconnected in series; and when the resonant converter circuit systemrequires only the first capacitor or the second capacitor to provideenergy, the controller is configured to: control one half of theresonant converter circuits to work in a first preset mode in which thefirst capacitor provides energy, and control the other half of theresonant converter circuits to work in a second preset mode in which thesecond capacitor provides energy.
 12. A resonant converter circuitcontrol method, applied to the resonant converter circuit according toclaim 1, wherein the method comprises: controlling, when a requiredoutput voltage is less than a preset voltage, switching statuses ofswitching transistors in a multi-level inverter circuit to reduce alevel difference of a square wave voltage signal output by themulti-level inverter circuit, so that the resonant converter circuitoperates within a preset range of a resonant frequency.
 13. The methodaccording to claim 12, wherein the multi-level inverter circuit is athree-level converter circuit; and the controlling, when a requiredoutput voltage is less than a preset voltage, of switching statuses ofswitching transistors in a multi-level inverter circuit to reduce alevel difference of a square wave voltage signal output by themulti-level inverter circuit comprises: controlling, when the requiredoutput voltage is less than a first preset voltage, the three-levelconverter circuit to output a square wave voltage signal with a leveldifference of E/2, wherein E is a voltage of an input power supply; andcontrolling, when the required output voltage is greater than or equalto a first preset voltage, the three-level converter circuit to output asquare wave voltage signal with a level difference of E.
 14. The methodaccording to claim 12, wherein the multi-level inverter circuit is afive-level converter circuit, the preset voltage comprises a secondpreset voltage, a third preset voltage, and a fourth preset voltage ofwhich a latter one is greater than a former one, and the fourth presetvoltage is less than a voltage E of an input power supply; and thecontrolling, when a required output voltage is less than a presetvoltage, of switching statuses of switching transistors in a multi-levelinverter circuit to reduce a level difference of a square wave voltagesignal output by the multi-level inverter circuit comprises:controlling, when the required output voltage is greater than or equalto the fourth preset voltage, the five-level converter circuit to outputa square wave voltage signal with a level difference of E; controlling,when the required output voltage is greater than or equal to the thirdpreset voltage and less than the fourth preset voltage, the five-levelconverter circuit to output a square wave voltage signal with a leveldifference of 3E/4; controlling, when the required output voltage isgreater than or equal to the second preset voltage and less than thethird preset voltage, the five-level converter circuit to output asquare wave voltage signal with a level difference of E/2; andcontrolling, when the required output voltage is less than or equal tothe second preset voltage, the five-level converter circuit to output asquare wave voltage signal with a level difference of E/4.
 15. Themethod according to claim 12, wherein there are two or more resonantconverter circuits, the resonant converter circuits share one bleedercircuit that is connected in parallel to two ends of an input powersupply, and the bleeder circuit comprises a first capacitor and a secondcapacitor that are connected in series; and the controlling, when arequired output voltage is less than a preset voltage, of switchingstatuses of switching transistors in a multi-level inverter circuit toreduce a level difference of a square wave voltage signal output by themulti-level inverter circuit comprises: when only the first capacitor orthe second capacitor is needed to provide energy, controlling one halfof the resonant converter circuits to work in a first preset mode inwhich only the first capacitor provides energy, and controlling theother half of the resonant converter circuits to work in a second presetmode in which only the second capacitor provides energy.